The role of free radicals in the pathogenesis of human disease has led to an increased interest in the study of free radicals and their reactions. Reactive oxygen metabolites can interact with cellular constituents, including DNA/RNA, proteins, and unsaturated lipids. Previous studies have suggested that hemoproteins may be involved in redox reactions which contribute to tissue and/or organ damage via reaction with hydrogen peroxide. In a recent report, nitrotyrosine levels have been associated with the presence of coronary artery disease. Thus, the determination and characterization of the heme-protein intermediates is important in understanding the mechanisms of these reactions.